A number of medical ailments are treated or treatable through the application of a magnetic field to an afflicted portion of a patient's body. Neurons and muscle cells are a form of biological circuitry that carry electrical signals and respond to electromagnetic stimuli. When an ordinary conductive wire is passed through a magnetic field, an electric current is induced in the wire. The same principle holds true for biological tissue. When a strong magnetic field is applied to a portion of the body, neurons are depolarized and stimulated. Muscles associated with the stimulated neurons contract as though the neurons were firing normally.
The use of magnetic stimulation is thus very effective in rehabilitating injured or paralyzed muscle groups. Further, the process is non-invasive, since magnetic fields easily pass through the skin of a patient. Apart from stimulation of large muscle groups such as the thigh or the abdomen, experimentation has been performed in cardiac stimulation as well. In this context, magnetic stimulation of the heart may prove to be superior to CPR or electrical stimulation, because both of those methods apply gross stimulation to the entire heart all at once. A magnetic stimulator can be used as an external pacer to stimulate each chamber of the heart separately in the proper sequence. Another area in which magnetic stimulation is proving effective is treatment of the spine. The spinal cord is difficult to access directly because vertebrae surround it. Magnetic stimulation may be used to block the transmission of pain via nerves in the back, e.g., those responsible for lower back pain.
Magnetic stimulation also has proven effective in stimulating regions of the brain, which is composed predominantly of neurological tissue. One area of particular interest is the treatment of depression. It is believed that more than 28 million Americans suffer from some type of neuropsychiatric disorder. These include conditions such as depression, schizophrenia, mania, obsessive-compulsive disorder, panic disorders, and others. Depression is the “common cold” of psychiatric disorders, believed to affect 19 million Americans and possibly 340 million people worldwide. Modern medicine offers depression patients a number of treatment options, including several classes of anti-depressant medications (Sari's, MAI's and tricyclics), lithium, and electroconvulsive therapy (ECT). Yet many patients remain without satisfactory relief from the symptoms of depression. To date, ECT remains the “gold standard” for depression; however, many patients will not undergo the procedure because of its severe side effects.
Recently, repetitive transcranial magnetic stimulation (rTMS) has been shown to have significant anti-depressant effects for patients that do not respond to the traditional methods. The principle behind rTMS is to apply a subconvulsive stimulation to the prefrontal cortex in a repetitive manner, causing a depolarization of cortical neuron membranes. The membranes are depolarized by the induction of small electric fields in excess of 1 V/cm that are the result of a rapidly changing magnetic field applied non-invasively.
It is now well established that both the left and right prefrontal cortex regions of the brain have strong communication links to Limbic System structures, which contain the “circuits” controlling mood and general behavior. The objective of rTMS is to provide stimulation to these circuits through a non-invasive, sub-convulsive technique, relieving the symptoms of depression without many of the negative side effects of ECT or medications. The principal reported side effect of rTMS is discomfort at the site of stimulation. This is caused by the depolarization of neuron membranes in the scalp and resulting scalp muscle contractions, which occur at the frequency of said stimulation. About 25% of patients report this problem to be at a level that is very uncomfortable. In general, the higher the power and the higher the frequency of stimulation, the more discomfort is reported. Higher power, however, has been shown to be necessary to stimulate deeper midbrain structures directly. High frequencies, (e.g. greater than 1 Hertz) have been shown to have an anti-depressant effect.
A considerable amount of study has been devoted to rTMS, yet the problem of scalp discomfort remains unexamined. For example, U.S. Pat. Nos. 6,198,958 and 6,571,123 are directed to methods of monitoring a patient's brain function during TMS. These references discloses a method for recording an EEG during TMS and for monitoring an MRI scan during TMS. The references also discuss the eddy currents induced in the metal electrodes by the TMS pulses which can cause burning of the patient's scalp by heating of the metal electrodes.
Another area for potential discomfort involved in magnetic stimulation (e.g. rTMS) concerns accurate placement of the coil on the proper location of a patient's head in a quick, easy, and repeatable fashion. The first time a patient receives rTMS treatment, the patient is subjected to a dosing and testing procedure to determine how strong a field is required and permissible and precisely where on the patient's head a physician and/or technician must locate the stimulator coil. Subsequently, it is desired to avoid blindly hunting for the same correct location, so a mark is made on the patient's head. Typically, the patient is provided with a swimming cap that is written on to indicate the correct location of the stimulator coil. However, from treatment to next treatment, the patient may not put the swimming cap on his head in precisely the same location or configuration. Another option practitioners have used is to mark the patient's head directly with a pen. Because this may require shaving the person's head in part or entirely, the patient may feel unhappy with this procedure.